This application relates to mechanisms and processes for drying products in a microwave field. More particularly, the invention disclosed and claimed in this application relates to an apparatus and method particularly suited for microwave vacuum drying, in batch sequences, of powdered or particulate materials in the pharmaceutical, chemical and food industries.
Microwave vacuum drying on an industrial scale has been used, among other places, in agricultural applications (for seed drying) and in related industries. Such processes normally involve drying products in a vacuum chamber containing a field of microwave energy. The vacuum reduces the boiling temperature of the product's solvent. The solvent is often water but can be a variety of other substances. The microwave energy raises the temperature of both the product and its solvent to the solvent's boiling temperature, and then provides the latent heat of vaporization.
The combination of microwaves and vacuum has been found to produce quick, low-temperature drying with little product loss and little chance for fire or explosion--the latter safety factor is especially important when powdered or dust-containing materials are dried. Additionally, the microwave vacuum drying process provides inherent solvent recovery.
In the past, microwave vacuum drying processes have been applied to a large variety of products. Such processes have, for example, been applied to seed drying, as disclosed in McKinney, U.S. Pat. No. 4,015,341 issued for a "Seed Drying Process and Apparatus", and to frozen materials, as disclosed in Bradbry, U.S. Pat. No. 2,513,991 issued for a "Process for the Desiccation of Aqueous Materials from the Frozen State". The process has also been applied to food products, as disclosed in Jeppson, U.S. Pat. No. 3,409,447 issued for "Treating Food Products with Microwave Energy and Hot Gas of Decreasing Humidity". Microwave vacuum processes have also been used in the pharmaceutical and food industries.
Although some batch microwave drying devices have been built and used in the past, and although batch devices have been used in the pharmaceutical, chemical and food industries, the use of such machines is not well developed. Many of the batch drying machines have been laboratory machines, not wholly suitable for high volume commercial drying. Continuous operations are often suitable in many food and chemical industries. However, batch operations are especially important in the pharmaceutical industry because of federally mandated requirements for records on each batch of pharmaceuticals prepared. Batch drying processes are also useful in the food and chemical industries for products produced in discrete segments or smaller amounts. Accordingly, a need exists for further development in the field of batch microwave vacuum drying in commercial quantities, especially in the pharmaceutical, chemical and food industries.
Vacuum drying operations have limitations different from conventional drying operation. For example, the solvent, often water, will condense at a lower temperature than normal due to the introduction of a vacuum. The lower condensation temperature will cause the walls and windows of the drying chamber to be covered with solvent condensate, which will accumulate in the chamber unless some means is provided to prevent condensation or to remove the condensate.
Use of a microwave drying process poses other problems, most stemming from the likelihood of uneven microwave distribution within the dying chamber. Microwaves can only penetrate the product to be dried to a limited depth, a depth that varies from about one quarter inch to four inches, depending on the product. To protect workers and operators of the drying apparatus, microwave devices must also have a complete metal enclosure; yet the field strength of microwaves becomes zero at the surface of a metal wall. The product must therefore be kept at least one quarter wavelength from the surface of the metal wall, a distance that is approximately 1.2 inches. The shape of the metal container also affects the field strength within the heating chamber. For example, a sharp metal point produces a high field strength at the point's tip. Microwave vacuum dryers therefore should be designed with smooth interior curves to produce few, if any, areas of extreme energy concentration.
In the past, the most common form of microwave vacuum dryers for batch processes have been lab structures with rotating trays. Such trays are usually circular and allow the product to be either loaded directly onto the tray or placed in a container that is set on the tray. Generally, rotating trays are difficult to load and unload, especially if the product is spread on the surface of tray, because the product must be spread evenly across the entire tray. For pharmaceutical or chemical powders, the maximum thickness that will allow even drying is one to two inches. Additionally, even drying throughout the product can be achieved only if the product is kept near the outer edge of the tray, since the movement of the product through the uneven microwave field is greatest along that edge.
A more suitable approach for batch microwave drying is the use of product container in the form of a tumbling drum. Such drum configurations have been attempted by the inventor, but have posed several problems. The entire drum must be transparent to microwaves, which for all practical purposes requires the drum be constructed of hard microwave-transparent plastics. Such plastics, however, make poor gaskets, meaning that difficulties arise in sealing the drum to contain the product. Additionally, a vapor exhaust point must be provided in the drum, and the most convenient location for that exhaust point is the axis of rotation. If a small opening is used, the opening will act as a venturi, increasing the velocity of the exiting vapor so that the vapor will flow from the drum at a high velocity. The high velocity vapor movement will usually pull powders or other granulated material along with the vapor. A larger opening will reduce the venturi effect, but will limit the capacity of the drum. Moreover, rotating drums need a microwave transparent rotating mechanism to effect the rotation.
As an alternative to rotating drums, microwave batch vacuum drying operations can be accomplished using a product container having internal agitators. Design of a structure with internal agitation posses a variety of difficulties, including the need for a thorough and even mixing with little fracturing of the product. The container design also should avoid corners that will trap the product to be dried and prevent standing water or solvent spots. Any container design must also allow easy loading and unloading of the product.
Control of a batch microwave vacuum drying process requires consideration of several factors. For example, microwaves are most often absorbed better by the product solvent than the product itself. The amount of energy absorbed by the product/solvent combination therefore decreases as the percentage of solvent withinthe product decreases. During vaporization of the solvent, the combined product/solvent temperature will remain relatively stable, since energy absorbed by the solvent will be used for the vaporization. However, as vaporization nears completion, the combined product/solvent temperature will begin a more rapid temperature change, if the microwave field is not changed to compensate.
Different products also have different drying requirements. For example, the maximum temperature that is acceptable for a product varies depending upon the product. Additionally, different products may require different degrees of solvent retained within the product; complete drying is rarely desirable.
One conventional form of control for drying processes is a simple elapsed time control that halts drying after a predetermined period. Of course, other more complicated control systems have been used as well. Elapsed time control, while effective, often does not provide the versatility necessary to precisely control the drying and drying temperature of products. This is especially true for pharmaceuticals, which require exact control of all the product's characteristics.